Virus-like particles (VLPs) make excellent platforms for displaying antigens in a highly immunogenic format. However, to date, VLPs have not been used to identify antigenic epitopes. In contrast, filamentous phage display is a useful technique for identification of antigenic epitopes, but these phage make poor immunogens. In this proposal, we describe a new platform based on VLPs of the RNA bacteriophage MS2, which serves both as a substrate for epitope affinity-selection and directly as an immunogen. Because affinity-selection and immunogen presentation are conducted on a single structural platform, we expect that selected epitopes will be more faithful molecular mimics of the target epitope. As a test of this idea, we use our system to select antigenic epitopes using an MS2 phage library. This library will be screened using monoclonal antibodies (mAbs) that recognize a well-defined linear epitope and a complex conformational epitope. In AIM 1, we will develop techniques to optimize the selection of epitopes with high affinity for the selecting antibodies. In AIM 2, we will assess the affinities of selected recombinant VLPs, measure their immunogenicity, and determine whether these VLPs, which are selected with mAbs, elicit similar antibody responses. We believe that this technology represents a new way to select epitopes in a highly immunogenic context that mimics their native conformation, and will have significant implications for vaccine development. Successful completion of these studies will lay the groundwork for selections of potential vaccines using, say, broadly neutralizing mAbs against infectious disease targets of interest (mAbs targeting Hepatitis C virus, Human Papillomavirus, & HIV are examples, but there are many other potential targets). Moreover, since we have shown that VLP-based immunogens can effectively elicit antibody responses to self-antigens, we could also use this technique to develop vaccine alternatives to therapeutic mAbs used to treat chronic diseases. PUBLIC HEALTH RELEVANCE: In this project, we will use a new phage display system, based on virus-like particles (VLPs), for vaccine discovery.